During the industrial production of vinyl aromatic monomers or other ethylenically unsaturated monomers there is usually a requirement for high temperature processing operations, such as purification by fractional distillation, and the performance of said operations at elevated temperatures can often cause unwanted thermal polymerisation of the monomer. The thus formed polymer is a problem economically, and can also give rise to safety and process efficiency issues which adversely affect the monomer processing operation.
Economic loss occurs because the yield of valuable monomer is reduced due to polymerisation of the monomer to form a polymer which is merely valued, at best, on its calorific content as a fuel, whilst the process efficiency and safety problems are associated with the tendencies of the polymer to foul surfaces such as heat transfer surfaces, and to cause blockages to flow through the equipment or to increase the viscosity of process streams, so that flux oil or valuable monomer has to be added in order to reduce the viscosity and thereby enable the process stream to be moved readily by gravity, or by forced flow such as by pumping.
In order to control such unwanted polymerisation it is common practice to employ an anti-polymerant composition in the process stream. These anti-polymerant compositions essentially fall into one of two categories, namely inhibitors and retarders.
Inhibitors are effective in preventing the formation of polymer, but they are substantially consumed in this process. This consumption can cause problems in situations where the inhibitor cannot be replenished, for example because of feed pump failure. Consequent to the inhibitor being consumed there is a loss of the means of control of the polymerisation and, thus, rapid polymerisation can then occur.
Retarders, on the other hand, may be less effective than inhibitors in limiting the amount of polymerisation, but they do have the advantage that they are not substantially consumed under their conditions of use. Retarders are, therefore, more reliable because they are longer acting and they provide some security in situations where unplanned circumstances arise.
The prior art discloses many different compositions for use in controlling the extent of the unwanted polymerisation of ethylenically unsaturated monomers. However, very few of the prior art compositions possess the desirable characteristics of a polymerisation retarder and those that do display such characteristics are also associated with other properties which are disadvantageous.
Thus, for example, DD-A-234020 teaches the use of compositions containing phenols, whilst U.S. Pat. No. 4,654,451 discusses nitrosophenolic compounds. Phenylenediamine compounds are disclosed in U.S. Pat. No. 4,774,374, whilst U.S. Pat. No. 4,252,615 and U.S. Pat. No. 4,086,147 are both concerned with the use of nitrophenols. SU-A-819078 and EP-A-550754 describe the use of quinine methides for this purpose, and nitroxyl compounds are the focus of JP-A-01165534, GB-A-1127127 and CS-B-260755. In addition, the use of quinones is considered in JP-A-5085964, which discloses employing p-benzoquinone as a retarder, whilst JP-A-63235390 teaches methods which are based on the incorporation of alkylated quinones, naphthaquinones and anthraquinones for the control of unwanted polymerisation.
However, none of these prior art compositions fulfils all the requirements of efficacy and ease of use which are the desired attributes of a successful composition for the control of the degree of polymerisation of ethylenically unsaturated monomers during their production and processing. Consequently many subsequent attempts have been made by other workers to improve upon these aspects by means of compositions which contain at least two of these known substances. Thus, for example, processes are disclosed in U.S. Pat. No. 4,929,778 which require the use of a combination of phenylene diamines and hindered phenols, whilst mixtures of nitrophenols and hydroxylamines are considered in EP-A-240297, nitrophenols and nitroxyl compounds in U.S. Pat. No. 5,254,760, quinones and nitroxyl compounds in US-A-2002/0037958, quinones and hydroxylamines in WO-A-96/41783, and CN-A-1962582 teaches a mixture of a quinone with a nitrophenol and a nitrite.
Nevertheless, despite these efforts, even these more sophisticated compositions fail to provide all the desired characteristics of a successful polymerisation retarder composition for use in the control of the amount of unwanted polymerisation under the process conditions which are typically encountered, and there remains a need for an improved polymerisation retarder composition, most particularly for use in controlling the amount of polymerisation which occurs during the production and processing of ethylenically unsaturated monomers.
The desired characteristics of such a composition include a low potential for harm to humans, as well as a high efficacy of control of the amount of polymerisation under the process conditions. The conditions which prevail during the industrial processing of ethylenically unsaturated monomers can involve the use of elevated temperatures, for example up to about 140° C., for extended periods, e.g. two hours or more, and with low or very low oxygen levels.
Furthermore, due to the complex nature of such processing plants, non-standard operating conditions of temperature, dwell time and oxygen content can occur from time to time, thereby resulting in greater than normal reliance on the polymerisation control composition. The non-standard conditions can also involve an interruption in the flow of the polymerisation control composition into the ethylenically unsaturated monomer process stream, which may occur, for example, because a distillation column needs to be run under total reflux for a period of time. In such an event, it is a requirement of the polymerisation control composition that its efficacy should not be quickly exhausted, but that it should continue to give control over the whole duration of the period of the non-standard operating conditions. Such long-lasting polymerisation control properties can be provided by a retarder composition, whereas inhibitors do not have such longevity of efficacy and can be found to become ineffective too quickly to be useful.
It has been established that the polymerisation retarder compositions of the prior art containing phenols such as methyl pyrocatechol, cresols or xylenols, or those containing phenylenediamine compounds such as N-isopropyl-N′-phenyl-p-phenylenediamine, are relatively ineffective at controlling the amount of polymerisation of ethylenically unsaturated compounds when there is low or very low oxygen content prevailing during the processing of ethylenically unsaturated monomers. The consequence of this poor efficacy is an unacceptable economic loss of the vinyl aromatic monomer due to unwanted polymerisation. Furthermore, whilst nitroxyl compounds such as 4-amino-2,2,6,6-tetra methyl piperidine-N-oxyl (amino tempo) and 4-hydroxy-2,2,6,6-tetra methyl piperidine-N-oxyl (hydroxy tempo) are known to be very effective components of compositions for controlling the amount of unwanted polymerisation of ethylenically unsaturated monomers at low and intermediate temperatures (e.g. up to about 100° C.), their efficacy decreases with increasing temperature such that, above about 120° C., they cease to control the amount of polymerisation and merely modify the characteristics of the polymer which is formed. Furthermore, it is found that these nitroxyl compounds are inhibitors rather than retarders. Therefore, nitroxyl compounds do not have the persisting action required for controlling the amount of polymerisation in the event of, for example, a period of total reflux operation.
Other chemical types, such as quinone methides and Mannich base compounds, can be used to provide non-toxic retarder compositions, but they are effective only at lower operating temperatures, such as up to about 110° C. When they are used at temperatures in excess of this they become ineffectual and the doses required are too high to be economically viable when compared to current commercially employed nitrophenol-based retarder compositions.
Nitrophenol compounds such as 2,4-dinitro-ortho-cresol (DNOC), 2,6-dinitro-para-cresol (DNPC), 2,4-dinitro-phenol (DNP) or 2,4-dinitro-ortho-sec-butyl-phenol (DNBP or “Dinoseb”) are known to be effective components of polymerisation retarder compositions for controlling the polymerisation of vinyl aromatic monomers under the above mentioned conditions. These nitrophenols are particularly effective as retarders and so they are the most frequently used class of material in commercial use for controlling the amount of thermal polymerisation of vinyl aromatic monomers in processing plants. However, these nitrophenols are toxic materials with potential to cause harm to humans and to the environment. Similarly, the nitrosophenolic compounds, such as paranitrosophenol, are found to be effective components of retarder compositions, but such aromatic nitroso compounds are solid products with poor solubility characteristics, and they are therefore difficult to handle; in addition, the toxicity properties of these compounds are such that they also have potential to cause harm to humans.
Commercial operations engaged in operating vinyl aromatic monomer process plants prefer not to handle and use toxic materials for health and safety reasons. Instead, they seek to employ non-toxic materials in order to obtain the very evident benefits of promoting the health and welfare of their employees and protecting the environment. Furthermore, in view of their undesirable toxicity properties, these nitrophenols and nitrosophenols are increasingly becoming subject to regulatory actions by national and international authorities in order to restrict or prevent their commercial supply. Consequently, it is likely that such nitrophenols and nitrosophenols might cease to be available for commercial use in polymerisation control compositions in the future.
WO-A-96/41783 discloses compositions and methods of use of specified quinones and specified hydroxylamines for inhibiting the polymerisation of vinyl aromatic monomers. However, the compositions disclosed in WO-A-96/41783 are characterisable as inhibitors and they do not possess the desirable longer term protection attributes of a retarder.
In view of the problems which are associated with the polymerisation control compositions of the prior art, there exists a need for new, effective and non-toxic, polymerisation retarder compositions, which specifically do not contain materials which are toxic to humans, for use in controlling the amount of unwanted polymerisation of ethylenically unsaturated monomers during their production and processing. Thus, the present invention seeks to provide new polymerisation retarder compositions which offer significant improvement over the prior art compositions in terms of efficacy in use, long lasting activity, and low toxicity.